1. Field of the Invention
The present invention relates to an optical device for use in an optical fiber communication system, and more particularly, to a composite optical device which is applicable to both a single star system and a passive double star system.
2. Description of the Prior Art
In a communication system, for example, a telephone network, an optical fiber line (hereinafter referred to simply as an optical fiber) is sometimes used as a transmission line for connecting equipment at telephone office to terminal equipment of subscribers. As such an optical fiber communication system, a single star system is known in which pieces of terminal equipment are connected to office equipment with use of optical fibers exclusive for respective terminal equipment.
As shown in FIG. 1, in the single star system, office equipment A1 to An is connected to subscribers' equipment B1 to Bn via optical fibers L1 to Ln exclusive for the respective terminal equipment B1 to Bn. Transmission light .lambda.1 is sent from the office equipment A1 to An to the terminal equipment B1 to Bn via the exclusive optical fibers L1 to Ln. As the single star system is simply constituted, operation management of the system is relatively easy. Moreover, the single star system has an advantage that information capacity per subscriber is large and that crosstalk will hardly occur. On the other hand, the single star system has a disadvantage that system construction cost is high. Therefore, at the present, the single star system is mainly offered for the service for business users.
As another optical fiber communication system, a passive double star system is known. As shown in FIG. 2, the passive double star system has an optical coupler/splitter E of an M-input/N-output (M and N are integral numbers and M &lt;N) interposed between the office equipment A (corresponding to A1 to An in FIG. 1) and the subscribers' terminal equipment B1 to Bn. In this system, the office equipment can be connected to the optical coupler/splitter E with M pieces of optical fibers Le1 to Lem, less than N, the number of pieces of terminal equipment stalled, and hence the system construction cost can be reduced. In FIG. 2, symbols r1 to rn represent branching/coupling terminals of the optical coupler/splitter E on t he terminal equipment side. The terminals r1 to rn are respectively connected to the subscribers' terminal equipment B1 to Bn via the optical fibers L1 to Ln.
By the way, it is necessary to search or locate a fault where characteristics of an optical fiber deteriorate, so that a proper action can be taken when mechanical or transmission characteristics of the optical fiber deteriorate in the optical communication system. For this fault search (monitoring of optical fiber line), an optical fiber line monitoring and testing device such as an OTDR (optical time domain reflectometer) using a back-scattering method is utilized. With a typical OTDR, an optical output pulse (monitoring light) from a semiconductor laser is led to an optical fiber to be measured, and back-scattered light which was scattered and returned in the reverse direction from the optical fiber is detected to measure a loss of the optical fiber.
The single star system having the fault search function is constituted, for example, as shown in FIG. 3. Also, the passive double star system having a similar function is shown in FIG. 4. In FIGS. 3 and 4, N pieces of optical fibers S for sending monitoring light are connected to an optical fiber line monitoring and testing device D. The monitoring light .lambda.2 propagating in each of the optical fibers S is led to a corresponding one of the optical fibers L1 to Ln of the optical communication system through a corresponding one of optical couplers C1 to Cn.
As mentioned above, the passive double star system has an advantage that the system can be constructed with a low cost, for example, a half of the construction cost of the single star system. On the other hand, it has disadvantages that the cost for system operation management is high, information capacity per subscriber is small, and crosstalk will easily occur.
At the present, only the services with lower charges and smaller information capacity such as telephone, low-speed data, broadcasting and distributing images are offered for general home users. Thus, from the short-term and economic viewpoint, it is desirable for the service for general home users to construct a passive double star system. In the meantime, it is expected that various information services via B-ISDN (Broadband-Integrated Services Digital Network) will be offered for general home users in the future. However, it is hard to offer such information services through the passive double star system. Therefore, it is more desirable to construct a single star system first from the longer term point of view.
As already explained referring to FIGS. 1 to 4, the single star system and the passive double star system are different in basic construction. Thus, in order to construct the single star system shown in FIG. 3 after construction of the passive double star system shown in FIG. 4, it is necessary to eliminate connection between the optical coupler/splitter E and the optical fibers Le1 to Lem and the connection between the optical coupler/splitter E and the optical fibers L1 to Ln in FIG. 4 and to remove the optical coupler/splitter E from between the office equipment A and the subscribers'terminal equipment B1 to Bn and moreover to connect the optical fibers L1 to Ln to the office equipment A1 to An as shown in FIG. 3. In this way, it requires cost and labor to construct the single star system after construction of the passive double star system.